The present invention relates to a tail wire cutting method and a bonding apparatus executing the method.
Assembly processes for ICs (integrated circuits) and other semiconductor devices include a wire bonding process for connecting semiconductor chips and lead frames with wires; and for this wire bonding step, wire bonding apparatuses and bump bonding apparatuses are used typically. In the wire bonding process, connections are made, as shown in FIG. 10, with connecting wires 12a between pads 3 on a semiconductor chip 2 and leads 4 of a lead frame 15, all of that being part of a work 14.
FIG. 11 shows bonding actions in each one of the steps in the above-described bonding process conventionally employed. In the following description, a bonding process taken in a wire bonding apparatus will be described.                (1) In step (a), a ball 5 is formed at the tip end of a wire 12, and a capillary 16 is moved above a pad 3 (first bonding point).        (2) In step (b), the capillary 16 is made to descend, and bonding is performed on the pad 3 (first bonding point). The ball 5 is pressure-bonded onto the pad 3 (first bonding point) to form a first bonding point 6 (pressure-bonded ball).        (3) In step (c), after bonding, the capillary 16 ascends away from the pad 3 (first bonding point) while leading out the wire 12 and then moves laterally to the position of a lead 4 (second bonding point).        (4) In step (d), after moving to the lead 4 (second bonding point), the capillary 16 is made to descend, and bonding is performed on the lead 4 (second bonding point). By this bonding, the wire 12 is pressure-bonded onto the lead 4 (second bonding point) to form a second bonding point 7.        (5) In step (e), after bonding to the lead 4 (second bonding point), a clamper 17 is opened, the capillary 16 is raised, and the tail wire 8 is led out at the tip end of the capillary 16.        (6) In step (f), after leading out the tail wire 8, the clamper 17 is closed and raised together with the capillary 16, whereby the tail wire 8 is cut away from the lead 4 (second bonding point).        
The detail of the tail wire cutting in the wire bonding process as described above is shown in FIGS. 12(a) to 12(c). FIG. 12(a) shows the conditions prior to cutting the tail wire, FIG. 12(b) shows the conditions after the tail wire has been cut normally, and FIG. 12(c) shows a case where the wire 12 has bent and deformation has developed when cutting the tail wire 8.
As seen from FIG. 12(a), prior to tail wire cutting, the lower end of the tail wire 8 that has thinned in a tapered shape is connected to the bonding end 7a that has been mashed so as to become thin in conjunction with the bonding of the connecting wire 12a at the end of the second bonding point 7 to the lead 4 (second bonding point). As shown in FIG. 12(b), when the clamper 17 is closed and the clamper 17 and capillary 16 are raised, the tail wire 8 is pulled upward. When that occurs, the tail wire 8 is cut between the lower end of the tail wire that has become thin and the bonding end 7a. 
However, depending on the bonding conditions, there are cases that the shapes of the tail wire end and the bonding end 7a have not become all that thin. In such cases, when the wire 12 is gripped by the clamper 17, and the tail wire 8 is pulled up, the tail wire 8 will not immediately be cut at the bonding end 7a; as a result, a large pulling force acts on the tail wire 8 and, after such a condition is reached that the tail wire 8 has been pulled and stretched, the tail wire 8 will be cut at the bonding end 7a. In this case, as shown in FIG. 12(c), the tail wire 8 that has been pulled and stretched snaps upward with a recoil force when cut, so that the wire 12 below the clamper 17, and the tail wire will bend in an S shape. Tail wire 8 bending is a problem, causing ball shape defects due to electrical discharges or the like during bonding to the next pad 3. Also, even if the ball is well formed, bent segments remain in the wire 12 inside the capillary 16 and between the capillary 16 and the clamper 17. Therefore, when bonding is performed with that wire 12 between the pad 3 (first bonding point) and lead 4 (second bonding point), the connecting wire 12a between the pad 3 (first bonding point) and lead 4 (second bonding point) will bend in an S shape as shown in FIG. 13. When the connecting wire 12a is not in a straight line shape and deforms in an S shape as shown in FIG. 13, problems occur that the connecting wires 12a come into contact with each other.
In view of the problems described above, various methods have been proposed for wire cutting so that bending deformations do not remain in the tail wire. Japanese Patent No. 2723277, for example, discloses that prior to gripping the wire 12 by the clamper 17 and pulling the tail wire 8 up, the positions of the capillary 16 and the clamper 17 are moved laterally so that the centerline of the up and down motion of the capillary 16 and clamper 17 coincide with the bonding end 7a, and then the wire 12 is gripped, and the wire 12 is pulled up and cut. In Japanese Patent No. 2969953, after opening the clamper 17 and leading the tail wire 8 out, the tail wire 8 is made to resonate by causing the capillary 16 to vibrate at the characteristic frequency of the tail wire 8, thus cutting the tail wire 8 by that vibration.
However, in the conventional art disclosed in Japanese Patent No. 2723277, even if it is possible to prevent the tail wire 8 led out of the tip end of the capillary 16 from bending, when the bonding end 7a is not sufficiently thin, a large pulling force will act on the wire 12 when the tail wire 8 is cut. As a result, the wire 12 would be snapped upward by recoil forces when cutting the tail wire 8, so that the wire 12 below the clamper 17 and the tail wire bend into an S shape.
In the conventional art disclosed in Japanese Patent No. 2969953, when the frequency that can be applied to the capillary 16 is around 30 kHz, in order for the tail wire 8 to resonate, the length thereof must be made from 1.3 to 3.4 mm. Thus, the problem is that this art cannot be used for short tail wire lengths required in today's high-speed bonding apparatuses.